Recent studies suggest that osteogenic bone morphogenetic proteins (BMPs) are required for the rapid bone formation that occurs in neonatal animals, and for bone remodeling, the most important function of the adult skeleton. It appears that BMPs 4, 5, 6 and 7 are not essential mediators of bone formation in adults, as mice lacking any one of these BMPs maintain normal bone mass. In contrast, mice lacking BMP2 are unable to maintain adequate bone formation after birth and suffer of spontaneous fractures that do not resolve with time. In our laboratory we are currently investigating the molecular mechanisms responsible for spontaneous fractures and lack of fracture repair observed in the absence of BMP2. Our current data suggest that the lack of BMP2 induces a structural phenotype that is responsible for the spontaneous fractures. Additionally, bones lacking BMP2 show an increased expression of inhibitors of the Wnt signaling pathway and an increased number of skeletal progenitor cells. In this proposal we hypothesize that BMP2 has a central role in the regulation of bone homeostasis and in the control of the intramembranous bone stem cell niche. We address this hypothesis in four specific aims. In Specific Aim 1 we rescue the BMP2 phenotype by crossing these mice with mice heterozygous for Dkk1, an antagonist of the Wnt signaling pathway and by administering LiCl, a chemical activator of the Wnt signaling pathway. In Specific Aim 2, by means of intravital microscopy we explore the location and the organization of the intramembranous bone stem cell niche. In Specific Aim 3, we investigate the effects of the presence and absence of BMP2 on the niche. In Specific Aim 4, we examine the effects of aging on the intramembranous bone stem cell niche and rescue the aging phenotype by steady expression of BMP2. During the initial Mentored phase of the award we will exploit Specific Aims 1 and 2. Specific Aims 3 and 4 will be developed during the Independent phase of the award. In summary, these studies will evaluate the role of BMP2 in the regulation of the intramembranous bone stem cell niche with the intent of providing important information for the development of novel dental and maxillofacial bone therapies. PUBLIC HEALTH RELEVANCE: Recent studies have shown that stem cells (undifferentiated cells able to differentiate into different kinds of cells) and stem cell niche (the place where stem cells reside) may regulate the homeostasis of an organ or a tissue. A dysregulation of the mechanisms by which stem cells differentiate into other cells may lead to an imbalance among cell types and therefore to diseases. Since our previous studies have identified BMP2 as a key factor for bone homeostasis, we believe that developing a greater understating of the way BMP2 regulates bone stem cells may provide important information for the development of novel dental and orthopaedic therapies.